A gold-running ship should be fast (S+L or better) and have good cargo (4 or higher). If it has a special ability that helps with getting gold, or which makes your ship harder to hit, so much the better, but this isn't mandatory. All you need is speed and cargo. And numbers; a gold strategy won't do well with only one or two ships, so watch your point values. The only generic crew you'll want is Helmsmen and Explorers, and use these sparingly, since they take up cargo space that you need for gold. The few named crew who help with gold-running are usually worth using. Examples are the crew who makes one treasure worth +2 when you unload it, and the crew who lets you swap a coin for a random treasure on another island.
A fighter needs good cannons (3-rank or better) and decent speed (S+S or higher). L-range cannons aren't that much better than S-range, so don't worry too much about cannon ranges. The more cannons a ship carries, the better, both for better offense and for longer survival in battle. A special ability that helps in combat is always worth it.
In terms of crew, every fighter has to have a Captain, or a named crew who gives the move-and-shoot ability; no exceptions. Helmsmen and Cannoneers are good investments for ships with low speed and so-so cannons, respectively. Named crew that enhance combat are usually good investments. The best abilities for fighting ships and crew are a +1 against all ships in other factions (the "world-hater"), the same-action-twice (SAT), and "kill a crew with a hit." The "hit two masts with one shot," the masts that don't fall with the first hit, and the sac captain are also popular and effective.
One-masted ships usually die too quickly in combat to make good fighters. If you must go to war with a single-masted ship, she should carry a Musketeer to double her potential for damage. Another way to make a small ship live longer is to keep her close to a large ship. If the enemy takes a shot at the little ship, that means he's ignoring the big one, which is good news for you, bad for him.
Keep in mind that, if you don't fill a fighting ship's cargo with crew, it can grab a treasure or two if the opportunity presents itself. Don't pass up a chance to ram and board, either, if the odds look good. Just don't get sidetracked from your mission of mass destruction.
For this tactic, you need one or more good fighting ships, plus a cheap, slow ship to be your seagoing tow truck. Don't tie up your fighters with towing; it takes them out of action for too long, and since you can't move faster than S with a ship in tow anyway, an otherwise useless ship will do the job just fine. Better yet, if you have a ship that can transport itself and a derelict home as soon as you explore it, you can steal more than one ship in the course of a game.
The real fun of stealing the enemy's ship is that you can repair it and give it orders. This demoralizes the enemy, especially if his own ship attacks him. Or use the captured ship as a gold runner while your own ships continue with the game plan.
You need at least three ships for a decent combo fleet, so point costs become critical. Four ships, two gold and two fighters, is better if you have the ships to do it.
An interesting variation is a fleet of ships, each of which is equipped for both gold and combat. Examples are the Spanish Santa Isabel and the pirate Darkhawk II (see my article on "Ships & named crew" for details). A couple of ships like this can be totally flexible, fighting in one turn and grabbing treasure in the next. It's the ultimate combo fleet.
Boarding is a good way to kill enemy crew, or a slow way to steal treasure - you don't choose which gold you get, so the enemy will dump his one-point coins on you if he can. The risk of boarding is that you can lose the action, which will cost you a crew or a treasure when you thought you'd inflict that fate on the enemy. Bear in mind that, even if you only gain one gold, it also makes the enemy poorer by one gold, so you're actually pulling ahead of him by 2.
A good boarding ship needs a lot of masts, so it can win its boarding rolls. It needs speed (S+L or better) to catch its prey, and it needs enough cargo spaces to store its ill-gotten gains. Special abilities can make or break a boarding ship. A +1 on boarding rolls is okay, but to really succeed, you need abilities like "cannot be pinned," "cannot be shot from S range," or "can board from S range without ramming." Some ships or crew can take a treasure at random from a ship you're touching. This is a much better bet than a boarding action if you have it; or you can do both and get two treasures.
As a special case, sea monsters and submarines do most of their work by ramming and/or boarding. Because most of them are rather slow, it can be hard for them to find targets. Send them to places where enemy ships have to go, like an enemy home island, or a wild island with more gold than one ship can carry. Then the targets will come to you.
Another example would be an American ship with "Diamond" Nelson Turner aboard; he can swap a treasure for a randomly chosen coin on a nearby enemy ship. A ship like this has to be able to find and carry some gold (Turner needs something to trade), and it must be fast enough to catch the enemy gold ships before they can get their cargoes home. This ship, too, probably needs a fighting escort, since it will quickly make itself unpopular with the enemy.
A third example is a fighting ship with a Chainshot Specialist. The ability to stop an enemy in his tracks for a turn can be good, if you know how to use it. What is your goal in chainshotting him? Do you want to stop him from running so you can overtake him? Do you want to stop a strong pursuer so your ship can make a getaway? There are many possibilities, but if your ship is ill-equipped to take advantage of them, a chainshotter won't do you much good.
A final possibility would be what I call the "fun fleet" - a set of ships chosen for whimsical rather than tactical reasons, which you then have to come up with a plan for. An example (which is technically illegal, but illustrates my point) is my Saratoga Fleet, made of the USS Saratoga (RV 073) and the USS Saratoga (DJC 093). WizKids didn't keep track of the ship names they'd already used, and made two very different ships with the same name. I turned their mistake into a fleet whose tactical purpose is to startle other players when I bring it to the table. It could win with a mix of fighting and boarding, but with a fleet like this, winning isn't the point. How would you deploy the Fox Fleet (Sea Fox, Swamp Fox, and Dark Fox), or the Deadly Sins Fleet (Pride and Greed's Hammer)? Use your imagination!
With several battles under your belt, you'll learn which styles of piracy suit you best, and which ships perform best for you. There is no perfect fleet; you'll always be experimenting with different ships and crew, if only to keep your enemy off guard.
Keep in mind that some ships are made for one purpose, and are very good at their intended role, but not for much else. Others can play any role you choose, depending on the crew you put on them. Some are so cheap that, even though they can't do much, anything they do will pay their way. And some ships are just inferior, or overpriced for their abilities. If you can't think of a role for one of your ships, it may be the ship designer's fault, not yours.
When all is said and done, the best course to victory on the high seas is this: